Device for measuring tibio-femoral force amplitudes and force locations in total knee arthroplasty

ABSTRACT

A probe used during a total knee arthroplasty for measuring forces and locations of their points of application and thereby moments includes two load sensitive plates t to be inserted in one joint-compartment of a knee joint each and each being provided with a top surface and a bottom surface. At least two load sensors may be situated on the top surfaces and/or the bottom surface of each load sensitive plate.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of International Patent ApplicationNo. PCT/CH2004/000361, filed Jun. 15, 2004, the entire contents of whichis expressly incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a probe for measuring force amplitudes,locations and moments of force.

BACKGROUND OF THE INVENTION

During a Total Knee Arthroplasty (TKA), a surgeon needs to balance thecollateral knee joint ligaments in order to ensure a stable artificialjoint. Conventionally, the balance assessment is based on the surgeon'sperception and experience by manually manipulating the knee joint.Assessing the ligament forces and moments acting in the knee “by hand”is subjective, and can lead to relatively large errors and repeatabilityproblems.

In European document EP 1 304 093 by GOUGEON, a probe is used whichallows the measurement of the compressive forces between each condyle ofthe femur and the tibial plateau. This probe comprises two load sensors,one for each condyle, and is attached to a plate-like support. Adisadvantage of this probe is that it is not possible to determine thepoint of application of the compressive force. The moments which areimportant for ligament balancing remain unknown.

SUMMARY OF THE INVENTION

The present invention is to provide a remedy for the above-discusseddisadvantages. It is an object of the invention to provide a probe thatallows measuring the amplitude and location of the compressive forcegenerated by each condyle. The present invention accomplishes theobjective set out above with a probe used during a total kneearthroplasty for measuring force amplitudes and force locationscomprising, a first and second load sensitive plate, each for insertioninto a joint-compartment of a knee joint and each having a top surfaceand a bottom surface. The first and second load sensitive plates haveadjoining inner lateral side walls and the plates are connected at theadjoining inner lateral side walls to each other. Also, the first andsecond load sensitive plates include at least two load sensors.

Advantages achieved by the invention are that the probe allows formeasuring in real time the force amplitude and location separately foreach condyle and therefore computing the moments acting on the kneejoint to better assess the ligament balance; and keeping the patella atits anatomical place during the measurement, which is closer to thepostoperative situation.

In one embodiment, the first and second load sensitive plates maycomprise at least three load sensors, each non-colinearly arrangedallowing measurement of the force amplitudes and locations with respectto two perpendicular axes, preferably one extending in a medio-lateraldirection and the other one extending in a antero-posterior direction.

In another embodiment, the probe further comprises a tibial base plate,providing a fixed and defined reference surface for the probe.

In a further embodiment, the probe further comprises a set of wedgesallowing a variation of the tibio-femoral gap according to the patientanatomy and ligament releasing procedure.

In another embodiment, each load sensor may comprise a bridge-shapedstructure and at least one piezoresistive sensor attached thereto. Thebridge-shaped structures may be configured such that each load sensorprovides measurement at one discrete point. Preferably, the discretepoint measurements of the load sensors are spaced apart relative to oneanother by a distance greater than 2 mm.

The measured parameters may be used as inputs to a computationalbiomechanical model of the knee joint acting as an assistive expertsystem.

In a further embodiment, instead of electrically connecting the loadsensitive sensors with the data processing instrument, (e.g. thecomputer) by means of cables, wireless telemetry may be used to transmitthe measurement signals of the load sensors to the data processinginstrument, e.g. the computer. This has the advantage of improving theergonomics of the probe, and the handling of the probe may befacilitated as well.

BRIEF DESCRIPTION OF THE DRAWINGS

The probe is explained in even greater detail in the following exemplarydrawings. The probe may be better understood by reference to thefollowing drawings, wherein like references numerals represent likeelements. The drawings are merely exemplary to illustrate the structure,operation and method of use of the bone plate and certain features thatmay be used singularly or in combination with other features and theinvention should not be limited to the embodiments shown.

FIG. 1 a perspective view of a human knee joint with a probe insertedaccording to one embodiment of the invention and a computer; and

FIG. 2 a perspective view on the embodiment of the probe shown in FIG.1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 represents a measuring apparatus 22 comprising a probe 1 situatedbetween the tibial plateau 25 and the two condyles 8, 9 of a human kneejoint 4. The measuring apparatus 22 may also comprise a data acquisitionand processing instrument 24 connected wirelessly or by means of cables20 to the probe 1 and a computer 23. The probe 1 may comprise two loadsensitive plates 2, 3, each having a top surface 6 and a bottom surface7. The bottom surfaces 7 may be in contact with the top surface of atibial base plate 28 lying on the tibial plateau 25 and each of the topsurfaces 6 may be in contact with one condyle 8, 9 of the femur 26. Eachload sensitive plate 2, 3 is situated in one knee-compartment and issubjected to the forces of each condyle 8, 9. The probe may furthercomprise a set of wedges allowing a variation of the tibio-femoral gapaccording to the patient anatomy and ligament releasing procedure.

FIG. 2 depicts a perspective view on an exemplary embodiment of theprobe 1 comprising two load sensitive plates 2, 3 each for one condyle8, 9 of a human knee joint 4. The two plates 2, 3 may have apolygon-like shape with rounded corners. The peripheries of the plates2, 3 are configured preferably so that the shape of the probe 1 isadapted to the shape of the tibial plateau 25. Each load sensitive plate2, 3 may have an inner lateral side wall, 14 a, 14 b, respectively. Thefirst and second load sensitive plates 2, 3 may be connected at theiradjoining inner lateral side walls 14 a, 14 b, allowing for easierinsertion of the probe into the tibio-femoral gap. The connectionbetween the plates 2, 3 may not extend the length of the adjoining innerlateral side walls 14 a, 14 b. Such a connection allows thetibio-femoral gap to be varied by means of the above wedges.Specifically, the two load sensitive plates 2, 3 may be connected toeach other by means of a flexible joining element 19 attached at the topsurfaces 6 next to adjoining inner lateral side walls 14 a, 14 b of theload sensitive plates 2, 3. Furthermore, there may be a gap between thetwo connected load sensitive plates 2, 3. Each plate 2, 3 may contain atleast two, and preferably three load sensors 10 that are situated on thetop surface 6, bottom surface 7, or in combination on both the top andbottom surfaces 6, 7 near the periphery of the plates 2, 3. In theexemplary embodiment shown in FIG. 2, the load sensors 10 are providedwith cables 20 for measurement signal transmission that are combined toa cable form 21 near the periphery of the first or second load sensitiveplate 2, 3 whereby each cable 20 is connected to the data acquisitionand processing instrument 24 (FIG. 1). At a gap between the innerlateral side walls 14 a, 14 b, the cables 20 of the load sensors 10situated on the first load sensitive plate 2 may be integrated in theflexible joining element 19. Instead of using cables 20 the measurementsignals may be wirelessly transmitted. Wireless telemetry may be used totransmit the measurement signals of the load sensors to the dataprocessing instrument, e.g. the computer. This has the advantage ofimproving the ergonomics of the probe, and the handling of the probe maybe facilitated as well.

The load sensors, preferably at least three, may be non-colinearlyarranged, allowing measurement of the force amplitudes and locationswith respect to two perpendicular axes, preferably one extending in amedio-lateral direction and the other one extending in aantero-posterior direction. Each load sensor may comprise abridge-shaped structure 30 (FIG. 2) elastically deformable with respectto the load applied onto the top surfaces 6 of the load sensitive plates2, 3. The elastically deformable bridge-shaped structures 30 may besituated near the periphery of each plate 2, 3 and spaced apart fromeach other, whereby at least two of the bridge-shaped structures 30 aresituated at an angle relative to each other.

The bridge-shaped structures 30 may include thick-film piezoresistivesensors 33 and provided with a central pillar 32 which is convexly,preferably spherically, shaped at the bottom. The piezoresistive sensors33 sense an electric resistance change, depending on the force applied.Arranging the piezoresistive sensors 33 in a Wheatstone bridge allowsconverting the resistance change into an electric signal measurable bymeans of the data acquisition and processing instrument 24 (FIG. 1).

In particular, the bridge-shaped structures may be configured such thateach load sensor provides measurement data for a discrete point.Preferably, the discrete point measurements of the load sensors arespaced apart relative to one another by a distance greater than 2 mm.The contact between the load sensitive plates 2, 3 and the tibial baseplate 28 or the wedges (not shown) occurs at the bottom of the pillarcenter 32 of each bridge-shaped structure 30. In this way, threereaction forces R₁, R₂, R₃ orthogonal to the tibial base plate 28 aregenerated at the pillars 32 of the bridge-shaped structures 30 when aforce F is applied on the respective load sensitive plate 2, 3. Thesereaction forces R₁, R₂, R₃ may be measured by the piezoresistive sensors33. The measured parameters may be used as inputs to a computationalbiomechanical model of the knee joint acting as an assistive expertsystem. Specifically, the amplitude and location of the initial load Fmay be computed using equations of mechanical equilibrium. According tothe measurement device 22 shown in FIG. 1 the amplitude and the locationx, y within the reference system 34 of the compressive force on theprobe 1 is generated by each condyle 8, 9 and may be computed by meansof the computer 23 using the values of the electric signals measured bythe data acquisition and processing instrument 24.

The surgeon may display the computed parameters, i.e., the amplitude andlocation of the compressive forces and the resulting moments, whichcharacterize the degree of ligament force balance in real-time onmonitor 27 of the computer. Relevant parameters may be computed duringflexion/extension of the knee joint 4 (FIG. 1).

Description of the Surgical Procedure:

Following a standard opening of the knee joint 4, a tibial precut isperformed in order to obtain a flat reference surface and enough roomfor the probe 1. The probe 1 is inserted into the knee joint 4 and themeasuring system, as well as the computer 23, is prepared for theacquisition, processing and display of the measurements. The amplitudeand location of the compressive contact force, as well as the moments ofeach femoral condyle 8, 9, are then measured in real-time at variousknee flexion angles. The computer 23 displays the raw measurements andan interpretation of these measurements is based on a computationalbiomechanical model of the knee joint 4 which acts as an assistiveexpert system. The ligamentous balance is then corrected according tothe measurements and to the biomechanical interpretation. Themeasurement and correction procedure is repeated until the ligaments arebalanced. Finally, the probe 1 is removed and the prosthetic componentsare inserted. Alternatively, the probe 1 can be used after the insertionof the femoral and/or tibial prosthetic component.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

1. A probe used during a total knee arthroplasty for measuring forceamplitudes and force locations comprising: a first and second loadsensitive plate, each for insertion into a joint-compartment of a kneejoint and each having a top surface and a bottom surface, wherein eachof the first and second load sensitive plates each include at least twoload sensors, and wherein the first and second load sensitive plateshave adjoining inner lateral side walls and are connected at theadjoining inner lateral side walls.
 2. A probe according to claim 1,wherein at least two load sensors are situated on the top surface of thefirst load sensitive plate and at least two load sensors are situated onthe top surface of the second load sensitive plate.
 3. A probe accordingto claim 1, wherein at least two load sensors are situated on the bottomsurface.
 4. A probe according to claim 1, wherein at least one loadsensor is situated on the top surface and at least one load sensor issituated on the bottom surface.
 5. A probe according to claim 1, whereinthe first and second load sensitive plate comprise at least three loadsensors each that are noncolinearly arranged.
 6. A probe according toclaim 1, further comprising a base plate configured to be in contactwith the tibia.
 7. A probe according to claim 1, further comprising aset of wedges.
 8. A probe according to claim 1, wherein each load sensorcomprises a bridge-shaped structure and at least one piezoresistivesensor.
 9. A probe according to claim 1, wherein measured parameters areused as inputs of a computational biomedical model of the knee jointacting as an assistive expert system.
 10. A probe according to claim 1,wherein wireless telemetry is used to transmit measurement signalsemitted by the load sensors to a computer.
 11. A probe according toclaim 1, wherein each load sensor provides measurement data for adiscrete point.
 12. A probe according to claim 11, wherein the discretepoint measurements of the load sensors are spaced apart relative to oneanother by a distance greater than 2 mm.
 13. A probe according to claim1, wherein connection between the first and second load sensitive platesis via a flexible joining element.
 14. A probe according to claim 13,wherein there is a gap between the connected first and second loadsensitive plates.
 15. A probe according to claim 13, wherein connectionbetween the first and second load sensitive plates is along a segment ofthe adjoining inner lateral side walls.
 16. A method of using a probeduring a total knee arthroplasty on a knee joint for measuring forceamplitudes and force locations comprising the steps of: a. performing atibial precut to obtain a flat reference surface and space for theprobe, the probe comprising first and second load sensitive platesconnected at adjoining lateral side walls and including at least twoload sensors; b. inserting the probe into the knee joint; c. measuringin real time amplitude and location of compressive contact force as wellas moments of each femoral condyle at various knee flexion angles; d.displaying the compressing contact forces and moments on a display; e.interpreting the compressing contact forces and moments based on acomputational biomechanical model of the knee joint; f. correctingligamentous balance according to the interpretation; g. repeating steps(c) through (f) until ligaments are balanced; and h. removing the probeand inserting prosthetic components.